Water-blocking, petroleum-based filling gel in optical fiber buffer tubes and cable serves two purposes. First, by completely filling all of the free space inside a buffer tube containing an optical fiber or fiber bundle, the filling gel blocks the ingress of water into the cable. Second, being a thixotropic material, the filling gel mechanically couples the optical fiber or fiber bundle to the buffer tube and prevents the optical fiber or fiber bundle from pulling back inside the buffer tube as the buffer tube is processed, as the cable is installed or otherwise handled, or as the cable is subjected to thermally induced dimensional changes due to environmental exposure. A totally dry cable eliminates the filling gel from inside a buffer tube, whether loose tube fiber or ribbon cable. In a totally dry cable, filling gel may be replaced by a water-swellable element, which may be a tape or a yarn carrying a water-swellable material. The water-swellable element, however, may not provide for sufficient coupling of the optical fiber or ribbon to the buffer tube, thus allowing the optical fiber or ribbon to pull back inside the tube/cable when the cable is installed or exposed to temperature extremes.
U.S. Pat. No. 6,970,629 and U.S. Patent Application Publication No. 2005/0213902 A1 disclose adhesively attaching a water-swellable element (or dry insert) to a buffer tube. Although this may assist in preventing the optical fibers or ribbons from retracting with respect to the buffer tube, it introduces its own problem in the manufacturing of the cable. More specifically, it is preferable that the water-swellable element have the ability to move with respect to the buffer tube prior to and during cooling of the buffer tube, so that the optical fibers or ribbons do not experience a column-like bending when the buffer tube contracts during the cooling. Such bending of the optical fibers or ribbons may result in optical signal losses because the optical fibers or ribbons may touch the inside wall of the buffer tube.
Indeed, it is common to tension the optical fibers or ribbons to induce a positive strain during the buffer-tube-extrusion process. In this manner, when the buffer tube material contracts on cooling, the positive strain in the optical fibers or ribbons compensates for some of the buffer tube contraction before the optical fibers or ribbons begin to absorb the contraction by the column-like bending.